Shortening start-up times and improving starting reliability while increasing the number of starts is one of many new requirements with respect to plant flexibility that has arisen as a result of the increased use of renewable energy sources such as solar and wind.
A major factor limiting the load output of an existing combined cycle power plant is the allowed pressure and temperature transients of the steam turbine and the heat recovery steam generator as well as the waiting time required to establish warm-up times in the balance of plant and the main piping system. These limitations may also influence the start-up capability of the gas turbine of a combined cycle plant by linking the start-up of the gas turbine with the start-up of the steam turbine.
A method of warming a steam turbine involves using main steam generated from the start-up of a gas turbine or auxiliary steam from other sources generated from within the power plant. This pre-warming is required even for small steam turbines in order to avoid differential temperatures between inner and outer walls of the steam chest, and within the rotor. Unless this is done before the unit is exposed to nominal steam system pressures and temperatures, temperature differentials may create excessive stress in the turbine and/or the turbine steam control valve(s).
Larger steam turbines typically include the step of rolling the turbine during pre-warming. If steam is used to pre-warm the turbine, this introduces further constraints on the pre-warming process by restricting the flow rate of the pre-warming medium. For example, is the turbine is being rolled during the pre-warming process, if the flow rate of the pre-warming medium is too high through the nominal steam path, the turbine may rolling-off the turning gear as it accelerates prematurely. However, lower pre-warming medium flow rate will increase the heat-up time.
US Patent Application No. 2004/0088984 A1 describes a method for operating a steam turbine within a Rankine cycle comprising several no-load or light load and further distributing steam to individual stages during idle or low-load operation. This is achieved by measuring an enthalpy difference across a steam turbine stage and controlling a bypass around the steam turbine based on the enthalpy measurement so as to minimise the enthalpy difference.
An alternative to steam pre-warming is discussed in U.S. Pat. No. 5,473,898. This solution, which is applicable only to combined cycle power plants where the gas turbine compressor is in operation, involves directing hot air bled from the gas turbine air compressor through the flow path of the steam turbine to pre-warm a steam turbine. As the compressed air is sourced from the gas turbine system, this solution, like the solutions discussed previously, links the start-up of the gas turbine with pre-warming of the steam turbine and therefore has only a limited effect on overall start-up time and further cannot be used to keep the steam turbine on hot standby.